Recovery of carbon dioxide from gaseous hydrocarbons



1939 ROSENSTEIN 2l82305 RECOVERY OF CARBON DIOXIDE FROM GASEOUS' HYDROCARBON'S Filed Jan. 24-, 1938 m UCQQE E ow .4 .6 Mol. CO2 per Mol. K2HPO4 Invenror Ludwig Eensiein By his Afforneg: i

Patented Dec. 5, 1939 UNH'ED STATS ATENT OFFICE RECOVERY OF CARBON DIOXIDE FROM GASEOUS HYDROCARBONS Application January 24, 1938, Serial No. 186,656

3 Claims.

This invention relates to the separation, by means of an absorbent agent, of volatile acids from hydrophobic liquids, i. e., liquids which are immiscible with water, and especially mineral hydrocarbon oils, which may be either normally gaseous or normally liquid. By the term volatile acids, I mean those gases which in water solution have an acid reaction, but which are released unchanged upon sufficient heating of the water. Carbon dioxide, hydrogen chloride, sulfur dioxide, mercaptans and hydrogen sulfide are the main gases of this type which are present in the hydrocarbon mixtures commonly encountered in refinery and industrial operations.

The process may be employed for the separation and recovery of these volatile acids from the liquid mixture, or may be employed to purify other liquids by the removal of said acids therefrom.

More specifically, the invention relates to the treatment of industrial liquids which contain volatile acids, particularly hydrogen sulfide, such as products from the destructive distillation of coal, and various petroleum distillates, as, for example, lubricating oils, fuel oils, kerosene, straight run or cracked gasoline, light naphthas, or liquefied normally gaseous hydrocarbons, etc., with an absorbent which permits the recovery of volatile acids, particularly hydrogen sulfide, in a concentrated form, and substantially free from hydrocarbons.

Hydrogen sulfide may be used as initial material for various end-products and it is, therefore, desirable, for my purpose, to obtain hydrogen sulfide as pure as possible. For example, if it is desired to burn hydogen sulfide to sulfur dioxide as in the process for producing sulfuric acid, methods using organic bases or solutions thereof to separate hydrogen sulfide are objectionable when the liquids from which I-IzS is separated contain or consist of hydrocarbons. Many of these hydrocarbons are appreciably soluble in such solutions and are sufliciently volatile to be released along with the hydrogen sulfide when the solution is heated. Combustion of the recovered gas can, therefore, lead to combustion products such as carbon monoxide, which are undesirable. Moreover, most of such organic bases are appreciably soluble in the hydrocarbon mixture being treated and necessitate further treatment of the latter to purify the mixture and to recover the bases. The hydrogen sulfide obtained by my process may also be utilized to produce sulfur, as by oxidation in a Claus kiln, or by reacting it with sulfur dioxide produced by combustion of a portion of the hydrogen sulfide.

I have tested the various absorbents commercially utilized and have found them deficient for my purpose in one respect or another. For example, using a solution of sodium sulphide as absorbent, the following reversible reaction takes place:

On boiling the solution after absorption of hydrogen sulphide to substantial completion, the re-.. action is reversed to the extent of only about 12% and consequently the absorption capacity of the solution on continuous operation was so small as to be impractical. On the other hand, a solution of sodium carbonate absorbs hydrogen sul- 15 phide according to the reversible reaction:

On heating the solution saturated with hydrogen sulphide, only a portion of the hydrogen sulphide was released, the other portion remaining in the solution, thus:

NaI-ICO3+NaI-IS; NazS+H2O so that sodium sulphide accumulated in the solution until the same condition was established as in the preceding case.

I have discovered that aqueous solutions of K3PO4, preferably concentrated, are suitable for. my purpose as they absorb hydrogen sulphide rapidly and evolve substantially pure hydrogen sulphide on reversing the absorption reaction as by boiling the solution at atmospheric, superatmospheric, or low absolute pressures. The reversal may be carried as far as desirable by continued boiling, and when such boiling is practiced at an elevated temperature, the regenerated solution may be cooled before returning to the absorption step. In case the solution is boiled at a low pressure and corresponding low temperature, cooling of the solution can be dispensed with prior to its return to the absorption step. The regenerated solution may be used in batch, intermittent or continuous processes.

A solution containing both K3PO4 and K2HPO4. may be utilized to carry out the absorption of hydrogen sulphide but I prefer to operate with a solution initially containing K3PO4 only, for while K2HPO4 in the initial solution results in a higher percentage of the absorbed hydrogen sulphide being evolved for any given time of recovery, it also decreases the capacity of the solution for hydrogen sulphide by limiting the amount of &PO4 which can be carried in the solution. I

have found that the effective hydrogen sulphide capacity of the solution is greatest for one which initially contains only K sPO4. Nevertheless, because of the more rapid regeneration, solutions initially containing both K3PO4 and K2HPO4 may be found useful, as, for example, in a two stage system in which the liquid is contacted in series.

Working with an aqueous solution of K3PO4, I have found that during absorption, KsPO x is converted to KZHPOl, as shown by the reversible reaction:

with respect to the K2IIPO4 formed during the 1 absorption and/ or added initially so that no substantial crystallization-takes place during absorptionmAt any temperature of operation, a too concentrated solution will deposit crystals. However, ifthezconcentration of the solution is permittedrto adjust itself by depositing crystals then theresulting concentration will be the desirahlemaximum.

I have found, contrary to all expectation, that the sodiumphosphates act quitediiferently from potassium phosphates. First, the maximum usable concentration of sodium phosphates is much less because of their lesser solubilities, and

' second, the reversal of the absorption reaction is not nearly as rapid or complete. This is probably due to a formation of sodium sulphide in the reverse reaction; thus:

Consequently, sodium phosphate solutions are not nearly as efficient as potassium phosphate solution, and cannot be considered as equivalents thereof.

The process "can be conducted at various temperatures and pressures. The contact between the hydrocarbon mixture and the absorbing liquid may be obtained by passing the two liquids into a mixing device, such as an agitator, permitting the resulting mixture to form two phases, and separating these phases by decantation, centrifuging, or any conventional method known to the art. A series of such mixers and separators may also be employed, and may be used in multiple extraction or countercurrent extraction processes. Alternatively, the twoliquids may be caused to flow countercurrently in an extraction column. To liberate the hydrogen sulfide from the absorbing solution a regenerator of the type shown in the U. S. patent to Bottoms, 1,733,901 may be utilized. "Water which is vaporized with the hydrogensulfidemay beseparated therefrom by a condenser at the top. of the regenerator, as .dis-

closed by the patent, thus-maintaining the initial concentration of the absorbing agent. substantially constant. i Ifdesired, watermay. be introduced from .an outside source at suitable intervals the, hydrocarbon mixture is given in Table I.

or continuously to maintain the predetermined concentration of the absorbent agent if the condensate is not returned to the regenerator. Since the hydrogen sulfide leaving the system is saturated with water, the water thus removed can be compensated for by introduction from an outside source.

Mercap-tans and other acidic gases may also be removed from hydrocarbons with the liquid phase potassium phosphate treatment of my invention.

Potassium phosphate will scrub out hydrogen sulquantity of scrubbing solution, and/or one of different alkalinity.

Carbon dioxide may effectively be removed from solutions of .liquid hydrocarbons by treatment with K3PO4 or KzHPOi or mixtures thereof. While KzPOr is more effective as a scrubbing medium than K2HPO4, the latter is more easily regenerated and therefore has proven to be the more economical'scrubbing agent for C02. C02

being a stronger acid than HzS can be selectively extracted from solutions containing both by using an amount of K2HPO4 not substantially in excess of that required to remove the CO2. This extraction may then be followed by one in which KaPOi is used to remove the H28. Fractionation of C02 and .528 can thus be efiected.

' I havediscovered that the extraction of acid gases such as H2S, CO2, etc., from liquefied nor-- mally gaseous hydrocarbons is more efiicient than a similar. extraction from the same hydrocarbons inwthe gaseous phase. This difference is illustrated in the attached drawing in which equilibrium curves are plotted for mol per cent (302.. .m both'liquid and gaseous propane against mol 002 per mol K2I-IPO4 in a scrubbing solution. As

will be noted the same amount of K2HPO4 can remove 002 from the liquid propane to result in a much lower CO2 content, than from the gaseous:

propane.

I have found that certain petroleum fractions contain aliphatic acids which are absorbed by the potassium phosphate solution, but cannot be removed therefrom by simple heating. In such.

situations it is desirable to distill the hydrocarbons and treat only the fraction which does not contain such'acids, as, for example, the fraction having a 100 C. end point. These acids may also be removed by subjecting the initial hydrocara bon mixture to a preliminary washing treatment with sodium hydroxide.

There are set forth hereinafter several examples of my invention which it is understood are exemplary only.

Example I .A number of test samples of a liquid hydrocarbon mixture consisting of about 29.9% propylene, 69% propane, and 1.1% H25 by weight were each separately subjected to single batch extractions in the liquid state at room temperature with different amounts of aqueous solutions containing 2.0, 1.455 and 1.0 grams mols, of K3PO4 per kilogram, respectively. The weight per cent of HzS remaining in each treated portion of lar hydrocarbon mixture, but containing 2.1% HzS by weight were treated with various amounts of the potassium phosphate solutions as described above in Example I. The results are Table I Percent by weight Grams KaPO; solution per Sample No. gram of hydrocarbon g gg g fi mlxture mixture Gramanols KaP04 per kilogram of solution 2. 0 1. 455 1. 0

Example II.-A number of samples of a simi- Gran1-rnols K3PO4 per kilogram of solution. 2. 0 1. 455 1.0

Example III.-To illustrate the application of the countercurrent method of extraction, a liquid hydrocarbon mixture of the composition described in Example I and an aqueous solution containing 1.455 mols K3PO4 per kilogram were continuously introduced into a two stage countercurrent treater in the ratio of 235 grams of solution to 1000 grams of hydrocarbon mixture. The percentage by weight of H28 remaining in the hydrocarbon is given in Table III:

Table III Percent by weight H28 in hydrocarbon mixture Stage I have found that treating the hydrocarbon in the liquid state results in a substantial improvement over the corresponding treatment in the gaseous or vapor state. The following example demonstrates the superiority of my method over the known vapor phase treatment:

Example IV.Four samples of the hydrocarbon mixture used in Example II (containing 2.1% HzS) were separately treated in the liquid state in single batch extractions with difierent quantities of an aqueous solution containing 1.455 mols K3PO4 per kilogram, and the per cent HzS remaining in each treated sample determined. Four other samples of the same hydrocarbon mixture were treated with corresponding amounts of thesarne potassium phosphate solution, but in the gaseous state. The temperature and pressure were almost the same for the treatments in the gas and in the liquid phases. The

per cent by weight HzS remaining in each treated sample is shown in Table IV:

Table IV Per cent by weight HzS remaining in hydro- Granis K3PO4 carbon mixture so u ion per Sample gram of hydrocarbon mixture Liquld Gaseous phase phase treatment treatment P I'hese data show that scrubbing liquid hydrocarbons produces decidedly better results than scrubbing the gas, especially in the situations in which a high degree of removal of H251 is desired.

With a View of presenting comparable data to show the effect of varying the treating conditions and concentrations all of the above examples pertain to propylene-propane mixtures. My invention may, however, be applied with success to other hydrocarbons which may be normally gaseous or normally liquid. For example, light petroleum fractions, such as butane, pentane or similar light fractions, petroleum naphtha, gasoline, kerosene, as well as natural gasoline, stabilizer bottoms, refinery recovery hydrocarbon gases and recovering gasoline may be treated in the liquid phase in accordance with the described process.

Example V.-A mixture of liquid propane containing 8.7 mol per cent CO2 was extracted with 150% by weight K3PO4 in the form of an aqueous solution containing 1.5 mols K3PO4 per kilogram. The 002 content of the propane mixture was reduced to .3 mol per cent.

Another mixture of liquid butane containing 9.2 mol per cent CO2 was extracted with 800% by weight of K2I-IPO4 in the form of an aqueous solution containing 1.5 mols K2HPO4 per kilogram. The CO2 content of the butane mixture was reduced to .6 mol per cent.

It will thus be noted that K2HPO4 is less efiicient as a scrubbing agent for CO2 than K3P04, but the ease with which KzI-lPOl is regenerated by steaming more than compensates for its lesser scrubbing ability. Thus, for instance, a K3PO4 solution containing 1.5 mols K3PO4 and .734 mol CO2 per kilogram solution was boiled until its volume was of the original volume. The CO2 content was thereby reduced to .726 mol. On the other hand, a similar solution containing 1.5 mols K2HPO4 and .405 mol CO2 was distilled to ,5 its original volume and the CO2 content was thereby reduced to .084 mol per kilogram solution.

While I have in the foregoing described in some detail certain preferred embodiments of my invention and some variants thereof, it will be understood that this is only for the purpose of making the invention more clear, and that the invention is not to be regarded as limited to the details of operation described, nor is it dependent upon the soundness or accuracy of the theories which have been advanced as to the reasons for the advantageous results attained. On the other hand, the invention is to be regarded as limited only by the terms of the accompanying claims, in which it is my intention to claim all novelty inherent therein as broadly as possible in view of the prior art.

.This, application is a continuation-in-part of my co-pending application Serial No. 25,947, filed June 10, 1935, forProcess for treating mineral oil. I

I claim as my invention:

1. In the process of recovering CO2 from normally gaseous hydrocarbons of less than 5 carbon atoms by treatment with an aqueous solution of potassium phosphate, the improvement con sisting of contacting said hydrocarbons while in the liquid statenwith the said aqueous solution containing tripotassium phosphate under conditions to maintain both, the material being treated and the treating agent, in the liquid state, thereby producing an aqueous solution containing absorbed CO2, and recovering the CO2 from the latter by steaming.

2. In the process of recovering CO2 from nor- Inally gaseoushydrocarbons of less than 5 carbon atoms by treatment with an aqueous solution ofpotassium phosphate, the improvement consisting of contacting said hydrocarbons while in the liquid state with-the said aqueous solution initially containingdiand tri-potassium phosphate under conditions to maintain both, the maof potassium phosphate, theimprovement con-' sisting of contacting said hydrocarbons While in the liquid state withthe said aqueous solution under conditions to maintain both, the material beingtreated and thetreating agent, in the liquid state, the potassium phosphate in said solution initially consisting, essentially 'of die-potassium phosphate, thereby producing. an aqueous solu- .tion containing absorbedCOz, and recovering the CO2 from the latter ,by steaming.

LUDWIG ROSENSTEIN. 

